pollen grains that fall on the stigma way or another, begin to germinate.The grain swells, pollen tube is formed from the vegetative cells.Thus, the germination starts.The shell breaks in the most delicate spot pollen tube.From the tip of the tube has special agents.They soften the fabric style and stigma.
core of generative and vegetative cells pass into the pollen tube.Generative cell divides and forms two sperm.Through the micropyle (a hole in the shell) ovule there is penetration of the pollen tube in the embryo sac.Inside the bag tube is broken and its contents poured out.
then merge with the egg of one of the spermatozoa.Thus, a zygote, then start giving the seed embryo.At the same it merges with the sperm of the second central core.This leads to the formation of triploid nucleus, rolling in later in the triploid endosperm, which is the secondary.
entire process described above is called "double fertilization in flowering plants'.
Antipodes and synergids (polar nuclei in the embryo sac) after completion of the process to dissolve and integument (cover) to form the seed coat.
Double fertilization in plants was discovered by Russian scientist S. Navashin.
During this process, the nucleus in the embryo sac is usually much larger than the sperm nucleus, which is due to the first finding in interphase (intermediate phase).At the same time the state and shape of the nuclei of the embryo sac may vary.Thus, in some Compositae they may be elongated, sometimes tortuous, non nucleoli.Generally, sperm are presented in the form of rounded interphase nuclei with nucleoli, in some cases having a structure unlike the female nuclei.
Double fertilization can be of two types, depending on the nature of the union of male and female nuclei.
So, if the process takes a dive premitoticheskom nucleus of the sperm in a woman's core, where the despiralization (unwinding) chromosomes.This union of two sets of chromosomes in interphase nuclei is carried out.
postmitotic double fertilization is the introduction of female and male nucleus, keeping envelope in prophase, which ends their union.Education interphase nuclei, which contain chromosome sets, occurs after the host first mitotic division in the zygote.
A double fertilization of the egg takes place in the fusion of two haploid (containing half the number of chromosomes) cores, so diploid zygote nucleus (with a full set of chromosomes).
Depending on the number of polar nuclei contained in the central cell is formed chromosome number of endosperm nuclei.
Double fertilization contributes to the manifestation of the dominant features in the endosperm of seeds (hybrid) endosperm from the father plant.With the penetration of the embryo sac a few pollen tubes, the process involves sperm from the first tube, in other tubes sperm degenerate.Double fertilization by two sperm is extremely rare.
During several embryological studies using various representatives of the angiosperms (Asteraceae, Ranunculaceae, Liliaceae) Navashin proved affiliation endosperm as the embryo to the sexual process in flowering plants.This phenomenon is characteristic only of representatives of the angiosperms flora.
It should be noted that the work Navashin it possible to clarify the origin of the endosperm.The implications of the findings of the scientist have been further developed.
